Rainout Assessment after a Release of Hazardous Liquefied Gas: Effect of Physical and Storage Parameters

Transcription

1 Rainou Assessmen afer a Release of Haardous Liquefied Gas: Effec of Physical and Sorage Parameers Valenina Busini 1, Renao Roa 1*, Federica i Vio 2, Sabaino iali, Robero Fiore 3 1 Poliecnico di Milano ip. Chimica, Maeriali e Ingegneria Chimica G. Naa via Mancinelli 7, 2131 Milano, Ialy, renao.roa@polimi.i 2 Tecnimon SpA esign HSE ep. 3 Saipem SpA - B.U. Onshore Snamprogei Cenre of Excellence - Loss Prevenion & Environmen ep. In process indusry i is common pracice o sore and handle dangerous subsances, which are gaseous a amospheric emperaure, as pressuried or refrigeraed liquids. In case of accidenal release, a uid in hese condiions gives rise o a wo-phase discharge. In such condiions, he liquid breaks up in an aerosol cloud, whose behavior in amosphere affecs dispersion disances. The physical phenomena occurring afer he release o he amosphere are mainly hree: expansion o ambien pressure, liquid aomiaion (break-up) and rainou. In paricular, he rainou fracion is a crucial parameer in quaniaive risk analysis and herefore models capable of esimaing wheher i occurs or no are of paramoun imporance. This sudy aims a emphasiing he inuence of some physical parameers and sorage relaed parameers on he rainou fracion, aking ino accoun coninuous and saionary releases of ammonia and LNG (Liquefied Naural Gas). These represen wo differen classes of subsances: gas liquefied by pressure (superheaed) and gas liquefied by refrigeraion (subcooled), which undergo o differen je break-up mechanisms. The resuls obained for hese wo compounds can hen be exended o he whole range of pressurised liquids/liquefied gases. A simple model has been developed o evaluae rainou occurrence, he simple approach here proposed (which has been validaed comparing is resuls o hose of oher simulaion models as well as o some available experimenal daa) allows o invesigae in a simple and effecive way he imporance of he differen physical and sorage parameers involved during rainou. 1. Inroducion angerous maerials ha are gaseous a ambien emperaure are commonly sored as pressuried or refrigeraed liquids. In case of accidenal release, hese liquids give rise o a wo-phase discharge. In such condiions, he liquid breaks up in an aerosol cloud, which behavior in amosphere affecs dispersion disances. The physical phenomena occurring afer he release o he amosphere are mainly hree: expansion o ambien pressure, liquid aomiaion (break-up) and rainou.

2 In paricular, he larges les can lead o he formaion of an evaporaing pool, while he smalles ones will say ino he gas cloud. Undersanding wheher a specific liquid release will lead o a pool formaion or no is of paramoun imporance o define he inle condiions for he cloud dispersion modeling leading o he safey disance esimaion. This is quie cumbersome since he physical phenomena occurring afer he release of a liquefied gas (namely, je expansion up o he ambien pressure, mechanical and/or ashing liquid break-up, and rainou) are no easy o model in deail. The rainou fracion is a parameer of crucial imporance in Quaniaive Risk Analysis (QRA) and herefore lumped parameers mahemaical models have been developed for mechanical and/or ashing liquid break-up, as well as for rainou (Cleary e al., 27; Wilox e al. 27). However, such models, while requiring dedicaed compuer codes (e.g., Wilox and Hol, 2) sill reain many approximaions and heir agreemen wih he available experimenal daa is quesionable. I follows ha for a firs-aemp esimaion of wheher rainou will occur or no (which is he main informaion required for seing up he subsequen simulaion of he amospheric dispersion of he ammable cloud) simpler correlaions would be desirable. Among all he pressuried and refrigeraed liquids in his paper he ammonia and he Liquefied Naural Gas (LNG) has been considered and he available experimenal daa of heir rainou have been used o validae a shor cu mehod for predicing rainou fracion. 2. Shor-cu Rainou Correlaion A simple correlaion o be used for predicing wheher rainou is expeced or no in a given siuaion should be able o accoun for he inuence of he main physical and sorage-relaed parameers inuencing he liquid break up and he subsequen les evaporaion (e.g., laen hea of vaporiaion, vapor pressure, liquid densiy and surface ension, -sie, wind condiion, ambien emperaure, air enrainmen). A simple correlaion has been developed o evaluae rainou occurrence, saring from he observaion ha rainou occurs when he uid released afer a loss of conainmen breaks ou ino les which ake more ime o evaporae (i.e. he evaporaion ime evap ) han o fall on he ground (i.e. igh ime, ). This consideraion leads o he following crieria for rainou exisence: evap evap > 1 < 1 ηrainou ηrainou > = (1) Boh he characerisic evaporaion ime and he characerisic igh ime depend on les diameer. In urn, his characerisic dimension is deermined by he mechanism ha deermines he liquid break up: shear sress (mechanic or aerodynamic breakage) or

3 liquid ash (hermodynamic breakage). The dominaing mechanism depends on he physical characerisics of he liquid as well as on he sorage condiions (e Vaull and King, 1992; Bricard and Friedel, 1998; Wilox e al. 27), which herefore deermine he characerisic le dimension and in urn he wo characerisic imes. evap By comparing hese wo characerisic imes (ha is, when = 1) i is possible o idenify a hreshold le diameer, lim, for rainou: les larger han his hreshold will rainou, and he rainou fracion, η rainou, will be roughly proporional o he difference beween he le diameer and he hreshold diameer, as follow: ( lim ) η rainou = (2) The evaporaion ime for he le can be easily evaluaed inegraing he maerial balance and he energy balance for he evaporaing le: evap 1 ΔH 2 evap ρ liq = 4 ( + sup ) v Sh AB ρ ρ h Nu k ( T T ) (3) is Sauer mean le diameer, ΔH evap where released liquid evaporaion enhalpy, ρ liq released liquid densiy, Sh Sherwood number, AB maerial diffusiviy, ρ air densiy, ρ P v ( T ) MW sup = is he le superficial densiy (MW RT v molecular weigh and R perfec gas consan), h vapor enhalpy, Nu Nussel number, k hermal exchange coefficien, T le emperaure, and T air emperaure. Equaion 3 shows ha he evaporaion ime depends from he le diameer boh direcly and indirecly, for he presence of Sh and Nu. The igh ime of he le can be obained by he inegraion of he momenum equaion for he le along horional and verical direcions, applying he following assumpions: 24 laminar condiions: C =, Re du, consan verical velociy: =, d

4 The igh ime for he le is insead evaluaed as follow: = 2 g( ρ liq ρ ) 18μ + u, (4) Where is he release heigh, g graviy acceleraion, μ he dynamic viscosiy, and verical componen of wind velociy u. Afer having defined he evaporaion ime and he igh ime for he le i is possible o evaluae heir raio, assuming ha: he le velociy ( u ) is consan, he wind direcion is horional ( u, = ), he le emperaure ( T ) is consan, he release direcion is horional. Solving he simple relaion evap =, he limi le diameer is deermined and hen, knowing he iniial le diameer, also he rainou fracion can be esimaed. The iniial le diameer is evaluaed choosing he smalles one beween he wo calculaed considering boh mechanical and aerodynamic uid break-ou. The wo correlaions used o evaluae he diameer of les originaed by mechanical ( mec ) and aerodynamic ( aer ) break-ou are he following: σ Wecri mec = (5) 2 u ρ f u, where σ is he surface ension, he criical Weber number We cri,=12.5, and u f le velociy afer je expansion. = ln (6) 3 3 aer E p where E p [ P v ( T s ) P ] ν s + [ P s P v ( T s )] ν s = ΔH evap (7)

5 is he parial expansion energy, P v is he vapor pressure, T s he sorage emperaure of he released uid, P he pressure a he end of he expansion, P s is he sorage pressure, and ν he specific volume of he released uid a sorage condiions. s 3. Resuls This simple approach has been validaed comparing is resuls o hose of oher simulaion models as well as o some available experimenal daa; in paricular varying he heigh of he release, he sorage emperaure, and he sorage pressure. Moreover, i was applied considering only a mean diameer, bu i would define a limi diameer also in he case of a disribuion of le diameers (i.e., les larger han he lim rainou); of course in his case, he formula should be weighed on he disribuion funcion. For he sake of example, Figure 1 and Figure 2 compare he ypical resuls obained using his simple correlaion for LNG and ammonia respecively, wih he resuls of one of he more deailed models available (Wilox and Hol, 2). We can see ha he simple correlaion is able o represen quie correcly he rainou fracion, and in paricular i is able o foresee he presence or absence of rainou when some physical or operaing parameer (he sorage pressure in his case) changes Rainou fracion rople diameer [mm] (o-lim)/o PHAST rainou fracion rople diameer P L=1, Sorage Pressure [am] Figure 1 Rainou fracion and characerisic les diameer for a 1 m heigh release from a ank a consan emperaure as a funcion of he sorage pressure - LNG

6 Rainou fracion rople diameer [mm] (o-lim)/o PHAST rainou fracion rople diameer P L =6, Sorage Pressure [am] Figure 2 Rainou fracion and characerisic les diameer for a 1 m heigh release from a ank a consan emperaure as a funcion of he sorage pressure - ammonia References Bricard, P., Friedel, L., 1998, Two-phase je dispersion. J. Haardous Maerials, 59, 287. Cleary, V.M., Bowen, P.J., Wilox, H.W.M., 27, Flashing liquid jes and wo-phase le dispersion. I. Experimens for derivaion of le aomisaion correlaions. J. Haardous Maerials, 142, 786. e Vaull, G. E., King, J. A., 1992, Similariy scaling of le evaporaion and liquid rain-ou following he release of a superheaed ashing liquid o he environmen, For Presenaion a he 85h Annual Meeing, Air & Wase Managemen Assoc., Kansas Ciy, MO, June Wilox, H.W.M., Harper, M., Bowenb, P., Cleary, V.M., 27, Flashing liquid jes and wo-phase le dispersion. II. Comparison and validaion of le sie and rainou formulaions. J. of Haardous Maerials, 142, 797. Wilox, H.W.M., Hol, A., 2, Unified ispersion Model, in: Technical Reference Manual 6., NV, London.